ATP and creatine phosphate (also called phosphocreatine or PCr for short) make up the ATP-PCr system. PCr is broken down releasing a phosphate and energy, which is then used to rebuild ATP. Recall, that ATP is ‘rebuilt’ by adding a phosphate to ADP in a process called phosphorylation. The enzyme that controls the breakdown of PCr is called creatine kinase.
How energy is transformed The ATP-PCr energy system can operate with or without oxygen but because it doesn’t rely on the presence of oxygen it said to be anaerobic. During the first 5 seconds of exercise regardless of intensity, the ATP-PCr is relied on almost exclusively. ATP concentrations last only a few seconds with PCr buffering the drop in ATP for another 5-8 seconds or so. Combined, the ATP-PCr system can sustain all-out exercise for 3-15 seconds and it is during this time that the potential rate for power output is at its greatest.
If activity continues beyond this immediate period, the body must rely on another energy system to produce ATP.